Neutralizing antibodies may be critical in the development of an effective vaccine against human immunodeficiency virus (Type 1) (HIV-1). However, approaches to date have failed to develop vaccines that induce neutralizing antibody responses against primary isolates of HIV-1. This failure appears to result from two viral envelope characteristics: low antigenicity and poor immunogenicity. How to maximize expression of epitopes on mature envelope and enhance immunogenicity therefore are critical questions for rational vaccine design. In recent studies, rhesus macaques infected with mutant SIV strains lacking specific glycosylation sites in and around the gp120 V1 variable region were found to develop antibody responses that potently neutralized not only the mutant SIV strains but also fully glycosylated wild-type virus. This indicates that it is possible to effectively elicit neutralizing antibody responses against a primary lentivirus. It further suggests that glycosyl-deficient mutants have enhanced immunogenicity and are potential vaccine candidates. The major goal of this project is to understand the molecular basis of the increased immunogenicity of glycosyl-deficient SIV envelope by defining and characterizing the antibodies comprising the neutralizing response. The applicants propose to study the antibody response elicited by glycosyl-deficient envelope at the molecular level by isolating and characterizing recombinant antibodies from phage-display libraries prepared from these monkeys. The investigators will characterize the binding of monoclonal antibodies (mAbs) and antisera to wild-type and glycosyl-deficient SIV envelope in detail. To further dissect the response and to characterize the specificities of antibodies important for protection, the applicants will perform passive transfer and viral challenge experiments. The present hypothesis is that the absence of specific carbohydrate increases exposure of a potent neutralizing B cell epitope, making the epitope more immunogenic. However, other explanations such as effects due to carbohydrate density on general immunogenicity cannot be discounted. The studies proposed should allow the origin of the enhanced immunogenicity of glycosyl-deficient SIV envelope to be tested. Understanding enhanced immunogenicity in the SIV model, will set the stage for knowledge-based development of more immunogenic candidate vaccines for HIV-1.